The Stirling thermodynamic cycle and heat engines employing same are well known in the art. Stirling engines typically utilize reciprocating pistons to transform externally introduced thermal energy into useful work. A known type of Stirling engine is the "free piston" type wherein a hermetically sealed engine housing contains an engine working fluid and freely moving pistons which are not mechanically connected either to one another or to an external drive. Thus, in the free piston Stirling engine, the reciprocating pistons move, in accordance with the thermodynamic cycle, under the influence of the working fluid dynamic forces.
Referring to FIG. 1, a prior art free piston Stirling engine 20 typically comprises a displacer piston 22 and a compression piston 24 mounted in end-to-end relation on a common axis 26 within a common, hermetically sealed engine housing 28. Pistons 22 and 24 are respectively mounted for reciprocation in cylinders 30 and 32. A working fluid moves between an expansion space 34 beyond the upper end of the displacer piston and a compression space 42 between the compression and displacer pistons through a heater 36, a regenerator 38 and a cooler 40 via tubes 41. The heater, regenerator and cooler each have an annular configuration and are successively disposed around the displacer cylinder. Work is derived from the engine by adapting an alternator or other generating device to the compression piston within housing 28. Where vibration is a consideration, such engines typically have a spring-mass damper, not shown, mounted external to the housing at either end of the engine along axis 26.
Multiple Stirling engines may be used in order to meet the power requirements of a particular application. In such applications, arranging the multiple engines in a compact design usually is an important consideration, particularly when separate spring-mass dampers are required. Additionally, each engine requires a separate connection to a source of heated fluid to be pumped through the engine heater. These multiple, separate heater connections add to the difficulty of achieving a desired compact design. Further, it would be highly beneficial, particularly from a manufacturing standpoint, that the individual Stirling engines be of a modular design, and thus an energy conversion system can be readily constructed utilizing the appropriate number of engine modules to satisfy the energy demands for a particular application.
A Stirling engine heater, such as diagrammatically illustrated in FIG. 1, typically comprises a plurality of heat exchange tubes, each essentially parallel to axis 26, through which the engine working fluid passes. A heated fluid is pumped through the void space between these tubes in order to introduce heat into the working fluid. Thus, the heated fluid, typically a molten metal such as lithium, normally contacts the wall of housing 28, as well as the outer surface of cylinder 30. Materials used in the constructions of housing 28 and cylinder 30 therefore must be of a character capable of withstanding the molten metal. Refractory materials, such as niobium-1-zirconium alloy, are most compatible with molten lithium. However, housing 28 and cylinder 30 may be required to withstand substantial pressures during engine operation, and refractory materials typically tend to be brittle and can lose their strength in contact with trace impurities typically found in engine working fluids. Further, refractory materials are not castable and must therefore be machined to the desired shape and dimensions. It would be preferable to cast the housing and cylinder in order to simplify and cost-improve the manufacturing operation. Additionally, refractory material parts are difficult to join together.
The class of materials referred to as superalloys, e.g. MAR-M247, are castable and possess sufficient mechanical strength to withstand engine operating pressures. Further, superalloys are compatible with the typical engine working fluids. However, superalloys are not compatible with molten metals, such as molten lithium, and for this reason are not appropriate for those engine parts subjected to prolonged contact with such heated fluids. Thus, prior art heater designs present engine material and fabrication shortcomings.
The heater as described above is positioned to heat the working fluid prior to its entry into expansion space 34. The ideal Stirling thermodynamic cycle consists of an isothermal expansion of the working fluid. This is not practically achievable since the working fluid is actually heated prior to its entry into the expansion space and after the expansion process is essentially completed. During expansion the working gas temperature drops in accordance with the inverse relationship between gas temperature and volume and unless heat is supplied during the expansion process this process does not occur isothermally. This resultant departure from the ideal Stirling cycle causes a decrease in engine operating efficiency. It is thus desirable to isothermalize the expansion portion of the cycle.